Modern electronic systems operate in an environment containing electromagnetic emissions. Such electromagnetic emissions may be generated from man-made sources, e.g., electric motors, communications and broadcast transmitters, lighting systems and other electrical systems. Electromagnetic emissions may also be generated from natural sources, e.g., atmospheric electric disturbances and precipitation static.
Many electronic systems are, to some degree, susceptible to interference from electromagnetic emissions, and to maintain consistent and dependable results, electromagnetic interference with such systems must be prevented. In addition, the electronic systems must not emanate electromagnetic emissions beyond an acceptable limit.
Electromagnetic emissions that interfere with or radiate from electronic systems can be controlled by the application of a shielding material around the electronic system and/or around electrical subassemblies or components within the electronic system. Materials which are useful as shielding materials are those materials capable of reflecting or absorbing electromagnetic emissions. Such capability depends on, among other things, the electrical conductance of the material. Conductive materials shield better than non-conductive materials.
In the past, electrical systems were shielded from electromagnetic interference by enclosing the system in a metal enclosure. However, the cost, weight, and availability of metals plus the production costs of manufacturing such enclosures have forced the industry to turn to plastic enclosures. Plastic enclosures tend to be lightweight, dent resistant, wear resistant, low cost, readily available and easily manufactured. Plastic by itself, however, does not shield against electromagnetic interferences.
To be effective for shielding electromagnetic interferences, a plastic enclosure must either incorporate or be coated with a shielding material, i.e., a conductive material. Presently, the most common method of shielding comprises the use of plastic enclosures to which a conductive coating has been applied.
Many techniques are known for applying a conductive coating to plastics. Vacuum metalization, flame spraying techniques, and electroless plating of the plastics are methods known for depositing a metal coating onto a surface of plastic. Such techniques tend to be costly, require long production times and may result in a metal layer having poor adhesion to the plastic substrate.
A simpler and less expensive technique, which is appropriate for many applications is the application of a coating of conductive paint onto the plastic surface by conventional spray techniques, e.g., spray painting. Such a conductive paint comprises a hardenable fluid matrix and a conductive filler, e.g., metallic particles. However, there are several drawbacks associated with such conductive paints.
The most significant drawback is that coatings of conductive paints have a higher electrical resistivity than all-metal coatings. Conventional conductive paints can presently provide a surface resistivity only as low as about 2 ohms per square per mil of thickness and hence are not useful in applications requiring lower resistivities.
The resistivity of coatings of conventional conductive paints tends to increase significantly when subjected to elevated temperatures for extended periods. This further reduces the number and types of applications to which such conductive paints can be used.
Many conventional conductive paints contain relatively large conductive particles which produce coatings having rough surface finishes. For applications which require smooth finishes, e.g., a decorative exterior finish on a cabinet, such rough coatings must either be applied to another surface, e.g., the interior surface of the cabinet, which is generally more difficult or must be sanded or polished to a smooth finish.